This invention relates generally to techniques used in geophysical well logging, and more particularly to new techniques for automatically processing dipmeter signals or displacement measurements obtained between these signals to produce more accurate dip and azimuth representations of subsurface formations.
A common method of measuring the dip angle and direction or azimuth of subsurface formations employs a dipmeter tool passed through a borehole drilled into the subsurface formations. This tool may apply any of numerous means to obtain geophysical signals representative of variations of a particular formation characteristic, such as its resistivity. One such tool is described in the paper: "The High Resolution Dipmeter Tool", by L. A. Allaud and J. Ringot, published in the May-June 1969 issue of The Log Analyst.
Dip and azimuth measurements representing the inclination of a formation characteristic or feature may be determined from dip-meter signals containing information representing the intersection of such a feature at three or more radially spaced points on the borehole surface. The displacement between two points intersecting a common feature may be determined, under favorable circumstances, by correlating pairs of the dipmeter signals, each having a similar response to the common feature. Two displacements between three different points determine the position of a plane. The position of the plane is conveniently expressed by its dip .theta., an angle measured from a reference (usually horizontal) plane and its azimuth .phi., an angle measured from a reference direction (usually true North). Typically, the dipmeter signals are recorded on computer compatible magnetic tape at the well site for later processing. The recorded signals are processed using any of several techniques. Manual, semi-automatic and fully automatic processing may be used with the automatic processing being performed with either analog or digital computers. When digital computers are used, a computer program is also required.
A computer program to perform the digital processing operations is described in a paper, "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tape" by J. H. Moran, et al and published in the July, 1962 issue of the Journal of Petroleum Technology. An additional computer program is described in the paper, "Computer Methods of Diplog Correlation" by L. G. Schoonover et al, pages 31-38, published in the February 1973 issue of Society of Petroleum Engineers Journal. Further, programs to process digitally-taped dipmeter data may be obtained from digital computer manufacturers, such as IBM.
Results from digital processing are normally presented in tabular listings as dip and azimuth measurements versus borehole depth. When desired, the individual displacements found between the correlated curve pairs which led to the dip and azimuth values may be also presented. Further, most such programs will provide the ability to vary both the length of the correlation interval and the step used to move this interval between each correlation sequence. For the next sequence, the same correlation length is used, but the actual interval correlated is moved by one correlation step length.
At each step or depth level, one sequence of displacements between various pairs of signal combinations may be obtained. A typical sequence includes at least two displacements but may include a round of up to six displacements in each sequence when four separate signals are employed, for example. When a round of more than two displacements in one sequence is obtained, the displacements may be combined into many more possibly different combinations, each combination corresponding to perhaps a different dip and azimuth measurement. Since only two related displacements are required, it is common practice to utilize only what appears to be the two best qualified displacements. All others are discarded without further consideration, thereby producing only one result per sequence. Further, little is retained as to the position of the sources or dipmeter pads corresponding to the utilized displacements.
When large numbers of measurements result, as from recent high resolution dipmeter techniques, tabular listings are usually augmented by graphic presentations of dip and azimuth representations. The graphic displays vary with the interpretation objective, depending upon whether the purpose is for stratigraphic or structural studies. Accordingly, relationships between the corresponding dip and azimuth measurements and their continuity with depth are considered in different manners.
For stratigraphic purposes, trends of adjacent dip measurements with depth are usually used to classify the measurements. For example, measurements representing a trend of rapidly increasing dip with depth will be considered separately from measurements representing a trend of rapidly decreasing dip with depth.
In the stratigraphic analysis, it is important that the azimuth of these dips must remain substantially constant and thereby represent the general direction of sediment transport or perhaps the probable direction of down dip thickening. Also, dipmeter results are combined in a given analysis from intervals corresponding to a given depositional or stratigraphic unit.
Graphic displays used for stratigraphic analysis often ignore the actual depths once the above dip versus depth trend for a given azimuth range qualifies a group of measurements. Further, since in many cases the actual dip angle is not important and only the dip azimuth is significant, the dip angle may be completely ignored in the graphic display. Such displays are designed to statistically determine the azimuth corresponding to a primary and perhaps a secondary direction of transport or deposition.
Graphic displays used in stratigraphic analysis are typically the azimuth frequency plot (no dip or depth representation) and the Schmidt net and the Stereonet (azimuth versus dip but still no depth representation). These nets and several variations thereof have known statistical characteristics in that they may enhance either low or high dip measurement point groupings. Note that in their use, the dip and azimuth value for each measurement is combined and represented by a point in these nets. A description of some of these displays and their application is given in the paper "Stratigraphic Applications of Dipmeter Data in Mid-Continent" by R. L. Campbell, Jr., published September 1968 in the American Association of Petroleum Geologists Bulletin.
Stratigraphic and structural analyses distinguish themselves in the type of information needed. In stratigraphic analysis, the dipmeter signals hopefully represent bedding planes within the boundaries of a given geological unit. These bedding planes have little, if any, regional extent. In structural analysis, a deliberate attempt may be made to mask out such sedimentary features in favor of enhancing the boundaries of the individual strata.
Short lengths (1 to 2 or 3 feet) of dipmeter signals are correlated to obtain stratigraphic information while long lengths (10 to 20 or 30 feet) of signals are often correlated to obtain structural information. While use of long correlation lengths to obtain structural dip has been standard practice for some time, there are certain disadvantages associated with this practice. One is that the use of long correlation lengths masks dip patterns needed for stratigraphic analysis, thus additional computations must be made using a short length to obtain stratigraphic information. Another is that most long correlation length techniques may be influenced by frequently occurring stratigraphic features having a common dip and direction, even though each such feature is less pronounced than the structural feature. Thus, the use of long correlation lengths does not assure obtaining accurate structural dip information. Yet another disadvantage is that current correlation techniques tend to ignore possibly objectionable effects of rotation of the dip-meter tool within the long correlation interval.
The preferred approach is to obtain the detailed information available only from short correlation intervals and then apply previously mentioned trend analysis to separate the stratigraphic and structural dips. However, as the correlation interval is shortened, the probability of obtaining a completely erroneous displacement increases substantially. The wrong peak on the correlation function produced in the correlation process may be used to determine the displacement. Such invalid displacements may be combined with valid displacements and produce an erroneous dip which add scatter and confuse valid trends or when systematically erroneous, may even appear as false trends.
As a compromise, longer correlation intervals than are actually desired are employed to artificially reduce this scatter to an acceptable level so that any valid trend which may be present might be found.
It is therefore an object of this invention to provide a technique to reduce the scatter in dip and azimuth measurements determined from short correlation intervals.
One technique which is employed to reduce scatter and find dip and azimuth trends is to average long intervals of dip measurements obtained from much shorter intervals. Unfortunately, the valid trends present only as short intervals may be masked completely by such an averaging process. Further, the resolution and position of the correct peak obtained by correlating short intervals tends to vary considerably, consequently, the corresponding displacements lack accuracy. Certain combinations of such displacements may compound the variation and introduce unacceptable inaccuracies in the resulting dip and azimuth measurements.
It is therefore an additional object of the present invention to provide a technique to improve the accuracy and reduce the scatter of dip and azimuth measurements without necessitating long interval averaging.
Some of the averaging techniques include a preliminary process of sorting or discarding apparently stray dips before averaging to prevent their contributing to the average. This process adds both time delays and expense to a process which already produces too few dips for many purposes. Further, some of the apparent strays may actually be part of a valid trend which was unfortunately just sampled infrequently. Both the discarding and averaging processes suppress such valid dips.
It is therefore a further object of the present invention to provide an automatic technique to improve the accuracy of dip and azimuth determinations without reducing the number of valid dips or discarding dips because they do not comply with some long interval trend.
When such averaging techniques are employed, the intervals to be averaged are often chosen arbitrarily such as every 100 feet or the like. Yet such zoning or sample grouping is an important factor in most statistical analysis. In some techniques, independent geological information is examined (usually manually) to select specific zones to be averaged. This latter process requires considerable time as well as accurate coordination of the depths of the geological information and the dipmeter information. This depth coordination may be a problem in deviating holes where the dipmeter information might not correspond to true depths. It would therefore be advantageous to have the determination of zones be made from the dipmeter data itself.
It is therefore a further object of the present invention to provide a technique for automatically zoning dipmeter information by analyzing the dipmeter information itself.
As previously discussed, these are prior art techniques for statistically analyzing either the dip or azimuth information for long interval trends. These methods usually employ polar chart representations to classify the dip and/or azimuth measurements. In these plots, the dip varies with distance from either the center or the edge of the plots and the azimuth varies with the radial distribution from the center of the plot.
However, when one considers the type of errors likely to take place in the correlation processes, particularly in deviated holes, it is desirable that any analysis not separate the dip from the azimuth values for the purposes of the analysis. The analysis should be able to detect any interrelationship between the dip and azimuth for the individual measurements. More particularly, the analysis should respect the fact that erroneous displacements can be concealed when expressed only as the resulting dip and azimuth measurements.
It is therefore a further object of the present invention to provide a technique for analyzing displacements and combinations of displacements rather than computing and analyzing the resulting dip or azimuth measurements.
Prior art methods do attempt to select only the best displacements or combinations thereof by assigning a quality rating according to the correlation process which determined the displacement. The best rated displacements are selected while discarding poor quality displacements. Yet the best displacements may be distorted or exaggerated due to failure of the signal source to maintain its proper position in the borehole, while poorer rated displacements may be obtained from sources in a much better position to produce more accurate displacements.
Therefore, it is a particular object of the present invention to consider the relative position of the signal sources in selecting the most valid displacements.
In accordance with these and other objects of the present invention, apparatus and methods are provided for automatically determining with a machine the most valid combination of displacements from a plurality of displacements and combinations thereof. These displacements may be obtained between pairs of geophysical signals derived from separate signal sources located on a dipmeter apparatus passed through a borehole penetrating subsurface earth formations. Displacements between pairs of geophysical signals may be produced by comparing the similarity of signal features for various displacements on said signals. When it is determined that these displacements are substantially devoid of closure error and thereby correspond to the same formation feature, the signal source most likely not to be in the proper position in the borehole is located and those displacements common to said source are nullified from determining the position of the formation features reflected in the signal features of said geophysical signals.
It has been discovered that for many types of dipmeter apparatus, the signal source or pad located nearest the top side of the borehole when the borehole is deviated substantially from the vertical, tends to lose its proper position in respect to the borehole wall. Further, it has been discovered that the type of focussing normally associated with these dipmeters electrically extends the effect of this pad, overcoming to a large extent the lack of contact with the borehole wall, and in effect, repositioning the pad on the borehole wall. However, the corresponding diameter measurement does not reflect the effective position and thereby produces displacements which are distorted or exaggerated. When no considerations for the above are made, the dips computed from a displacement combination which includes displacements between signals obtained from such floating pads are also exaggerated. By locating the signal source most likely not to be in the proper position and disqualifying or nullifying those displacements associated with this source, particularly when planarity errors are known to exist only those displacements remaining may be selected as the most valid displacements.
In one form of the invention, a closure error is computed and if a substantial closure error is found, it is assumed that one or more displacements correspond to different formation features and the degree of distortion or exaggeration from a planar formation feature cannot be determined. However, if little closure error is found it is assumed that all the displacements used in the closure computation correspond to substantially the same formation features; therefore, planarity error, distortion, or exaggeration may be evaluated.
In one aspect of the invention, the actual position of the formation feature is compared with the expected position of the formation feature on a signal derived from a given source. These positions are determined from the given relationships combining related displacements. The displacements corresponding to the largest difference between the expected and actual positions are considered to be the most exaggerated or distorted and therefore disqualified as valid displacements.
In another aspect of the invention, the source most likely not to be in the proper position is located. The displacement relationships specific to that source may then be used to determine the degree of distortion or exaggeration. If this degree exceeds a given range it implies that the most likely source not to be in the proper position was in fact out of position. The displacements associated with a signal obtained from this source may on one hand be disqualified from further consideration as valid displacements or, on the other hand, corrected to eliminate the distortion. When the above technique is applied in highly deviated holes to determine those displacements which are valid and may therefore be combined as possible corresponding displacements, and these possibly corresponding displacements are used in a further technique, a substantial improvement in dips determined from the combination of techniques is obtained.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.